Probable reason/Recommendations

1. Contaminated heater exchanger

   
   When the exchanger is heavily contaminated a mechanical cleaning may be required. In this case it should be conducted manually by disassembling the consol.

2. External exchanger contaminations.
   
   In this situation an external cleaning is required, mechanically or using chemical products, like Verylube Engine surface cleaner

3. Airlocking of the cooling system.
   
   Just purge air from the system.

4. Thermostatic element malfunction.
   
   Change thermostatic element.